After 19 years, Stephen Sondheim’s most terrifying villain is back from the dead. No, not Sweeney Todd and not those bloody Assassins. It’s Fosca who lives again, in this CSC revival of “Passion” — although mad genius Sondheim and his fellow provocateur, book writer James Lapine, apparently envisioned this monstrous creature as a romantic heroine. The thesps aren’t required to play musical instruments in John Doyle’s sleek production, but given their demanding character roles, it’s imperative that they should act as well as sing. So give thanks the wonderful Judy Kuhn is on board.

Sondheim took this gothic narrative from a 19th-century Italian novel about a young military officer named Giorgio (Ryan Silverman, in good voice) who is torn from the arms of his married mistress, Clara (Melissa Errico, positively luscious), and assigned to a remote provincial outpost outside of Milan. Giorgio’s physical beauty and sensitive soul attract the attention of his commanding officer’s sickly and emotionally unstable ward, Fosca (Kuhn), who becomes so consumed with desire that she stalks Giorgio like a predatory animal.

Doubling as designer, helmer Doyle meets the challenge of setting the ungainly CSC stage by stripping everything down to basics: black brick walls, black marbled floor, heavily gilded mirrors, wine-red draperies, and really dark lighting. In this stygian setting, a small, well-drilled ensemble of golden-throated soldiers make a swell Greek chorus, and a couple of standout perfs — from Stephen Bogardus as commanding officer of the post, and Tom Nelis as medical officer — fully enlighten auds on day-to-day life in this rigid military society in which Giorgio finds himself.

Theatergoers don’t really need to see a solid set of the parade grounds where the soldiers march on review. Or the mess hall where they gossip like girls.  Or the train station or the dueling field or the billiards room or all the rest of it. We’re only here for the music, anyway.

In keeping with the romantic tradition, Sondheim’s luxuriously harmonic and unabashedly operatic score pours out in a continuous flow of melody. Flawlessly played by a small orchestra under the musical direction of Rob Berman, this intoxicating soundscape largely consists of a musical duel between the radiant romanticism of Clara’s tender love and the frightening ferocity of Fosca’s pathological obsession.

“So much happiness / so much love” is how Clara speaks of her overflowing joy as she clasps Giorgio in her arms. The soaring sound of her musical motif defines the high romantic period, when poets swooned over the notion of a true and perfect love. Errico’s delicate beauty and crystal-clear soprano make her ideal for the role, even if she can’t manage a convincing display of ardor for Silverman’s well-sung but physically rigid Giorgio.

If Clara is romanticism in its purest form, then Fosca is gothic melodrama at its most decadent. If Clara’s sound is light and crystalline, the music that defines Fosca is dark and brooding and quite fierce.

Sondheim’s genius has been to translate Fosca’s most insane manifestations of love into the most beautiful musical movements. Her stunning declaration of loneliness — “I read to live” — is as pitiful as it is frightening. And as her obsession intensifies — with a chilling definition of “love as permanent as death / implacable as stone” — her voice becomes all the more exquisite, her madness all the more terrifying.

Amazingly, Kuhn stirs feelings of compassion for the repulsive Fosca. Her forthright musical delivery exposes the insatiable needs of this ravenous succubus. But the warmth in her rich, mellow voice is a kindness, revealing the suffering behind the mad acts of a desperate woman.

Only the ending of this strange chamber piece presents a problem, as it did for audiences in the original 1994 production helmed by Lapine. Seriously, are audiences really supposed to stand up and cheer when Giorgio perversely rejects Clara as selfish and superficial, and embraces Fosca’s feverish passion (“Love without reason / Love without mercy” … “a disease that would cripple us all”) as the purest expression of love? Surrendering to love may be liberating — but surrendering to insanity is insane.

(CSC Theater; 199 seats; $80 top)

A Classic Stage Company production of a musical in one act with book by James Lapine and music and lyrics by Stephen Sondheim, based on the film, “Passione D’Amore,” directed by Ettore Scola.  Directed and designed by John Doyle.

Costumes, Ann Hould-Ward; lighting, Jane Cox; sound, Dan Moses Schreier; makeup, Angelina Avallone; orchestrations, Jonathan Tunick; music director, Rob Berman; music coordinator, Seymour Red Press; production stage manager, Adam John Hunter.  Opened Feb. 28, 2013.  Reviewed Feb. 22.  Running time:  ONE HOUR, 45 MIN.

Colonel Ricci —  Stephen Bogardus

Lieutenant Barri — Jeffry Denman

Clara — Melissa Errico

Private Augenti — Jason Michael Evans

Lieutenant Torasso — Ken Krugman

Fosca — Judy Kuhn

Sergeant Lombardi — Orville Mendoza

Doctor Tambourri — Tom Nelis

Major Rizzolli — Will Reynolds

Giorgio — Ryan Silverman